A hot-rolled steel sheet is usually manufactured by rolling so that variation in the thickness of the steel sheet along the longitudinal direction is minimized in one coil (one strip of the hot-rolled sheet). Thus, a hot-rolled steel sheet is strip-shaped and has a uniform thickness along the longitudinal direction. After hot rolling, pickling may be performed on a hot-rolled steel sheet, if necessary. In the following description, a hot-rolled steel sheet or a product made by slitting a hot-rolled steel sheet in the longitudinal direction will be referred to as a “steel strip”.
Such a steel strip is used, for example, to form an oil-well cleaning pipe. An oil-well cleaning pipe is manufactured by successively joining a plurality of pipe bodies, which have been made from the steel strips, to each other by welding, or by joining the steel strips to each other by welding and then performing a pipe-forming operation. The oil-well cleaning pipe is usually manufactured so that the diameter and thickness decrease toward one end. The reason for manufacturing the pipe in this way is to reduce the suspended weight. An oil-well cleaning pipe is coiled around a reel and transported to a site. At the site, the oil-well cleaning pipe is uncoiled or coiled as necessary.
When a plurality of pipe bodies are made from steel strips having a uniform thickness along the longitudinal direction and a long oil-well cleaning pipe is manufactured by butt-welding the plurality of pipe bodies and if the oil-well cleaning pipe is designed so that the diameter decreases toward one end, steps are formed at butt-joint portions of the pipe bodies. Such an oil-well cleaning pipe has a problem of a short lifetime, because a crack is likely to occur at a step of a butt-joint portion while the pipe which is coiled around a reel is uncoiled and coiled repeatedly.
When an oil-well cleaning pipe is manufactured by butt-welding a plurality of steel pipes made from steel strips having the same thickness, the suspended weight of the oil-well cleaning pipe is increased, and therefore it becomes necessary to increase the strength by, for example, increasing the grades of the material of the steel pipe. Moreover, because the weight of the entirety of the oil-well cleaning pipe is increased in this case, a problem occurs in that it is necessary to reduce the length of the oil-well cleaning pipe. Furthermore, an oil-well cleaning pipe may be transported along a road having a weight limitation of transport vehicle or the like, and it is preferable that an increase in the weight of the oil-well cleaning pipe be suppressed also for this reason.
A steel strip described in Patent Literature 1 is an example of existing technologies for addressing such problems. The steel strip described in Patent Literature 1 is a steel strip having a thickness that changes at a constant gradient along the longitudinal direction. In other words, the thickness of the steel strip gradually decreases at a constant rate from one of the longitudinal end portions to the other longitudinal end portion.
In the case of manufacturing an oil-well cleaning pipe, which is a long steel pipe, by joining steel pipes made from steel strips described in Patent Literature 1 to each other by successively butt-welding the steel pipes, the steel strips are manufactured so that the diameter of the tail end portion of a first steel pipe (on the small diameter side) and the diameter of the head end portion of a second steel pipe (on the large diameter side) becomes the same as each other, and the tail end portion of the first steel pipe and the head end portion of the second steel pipe are butt-welded. By repeating such a process, a long steel pipe (oil-well cleaning pipe) can be manufactured. Thus, it is possible to manufacture a long steel pipe that has a thickness decreasing toward one end and that does not have steps at connection portions (butt-welded portions).
Patent Literature 2 also describes a steel plate having a thickness that varies along the longitudinal direction. However, the steel plate described in Patent Literature 2 is a thick steel plate that is not to be coiled and that is not a steel sheet used as a coil or as a steel strip. In other words, it is different from a metal strip to which the present invention is related.